1. In software, a misfeature that becomes understandable only in
historical context, as a remnant of times past retained so as not to break
compatibility. Example: the retention of octal as default base for string
escapes in C, in spite of the better match of
hexadecimal to ASCII and modern byte-addressable architectures. See
dusty deck.

2. More restrictively, a feature with past but no present utility.
Example: the force-all-caps (LCASE) bits in the V7 and
BSD Unix tty driver, designed for use with monocase
terminals. (In a perversion of the usual backward-compatibility goal, this
functionality has actually been expanded and renamed in some later USG Unix
releases as the IUCLC and OLCUC bits.)